The present invention relates to a transmission antenna directivity control scheme for receiving a reverse link radio wave by a plurality of reception array antenna elements and controlling the directivity of a forward link transmission signal to be transmitted from each of a plurality of transmission array antenna elements in accordance with the reception signal.
The CDMA (Code Division Multiple Access) scheme can increase the subscriber capacity and is expected as a radio access scheme for a next-generation mobile communication cellular system. However, on the base station receiving side, another user signal simultaneously accessing using the same carrier causes interference, and on the mobile station receiving side, a signal transmitted to another user causes interference. To remove such interference, an array antenna is used.
In an array antenna, signals are transmitted/received by a plurality of antennas and subjected to weighting and combining using complex numbers to form directivity beams while controlling the amplitude and phase of the reception signal of each antenna, thereby suppressing interference with another user. An example of such array antenna control scheme is a multi-beam scheme.
FIG. 7 shows a conventional transmission directivity control apparatus using a multi-beam scheme. In this multi-beam scheme, a signal is received by N (N is an integer of 2 or more) antenna elements 1-1 to 1-N laid out in proximity in a reception array antenna section 1 and converted into digital signals by A/D converters 2-1 to 2-N arranged in an A/D conversion section 2 in correspondence with the antennas.
In a reception beam forming section 3, the reception signals output from the A/D conversion section 2 are multiplied by a weight coefficient, which is calculated in advance, by multipliers using M (M is an integer of 2 or more) beam formers 3-1 to 3-M for fixed beams. Next, the signals after multiplication are combined to control the phases and amplitudes, thereby realizing reception of beams formed in specific directions.
The M fixed beams are laid out to cover a predetermined spatial area (e.g., a sector) as uniform as possible. FIG. 4 shows, as multi-beam patterns, a case wherein a xc2x190xc2x0 area is covered using six quadrature multi-beams A to F. The quadrature multi-beams are formed such that the peak position of each multi-beam corresponds to the null level of other corresponding beams.
On the receiving side, a beam power detection section 4 measures the power of the output from each of the beam formers 3-1 to 3-M and notifies a beam output selecting/combining section 5 of each reception power together with a beam number. The beam output selecting/combining section 5 selects and combines one or more beams that exhibits high-level reception powers from the reception powers and outputs the combined beam. When the multi-beams shown in FIG. 4 are used, the reception gain degrades by about 4 dB from the beam peak near the intersection of two adjacent beams. Hence, for the signal of the desired radio wave that arrives from the intersection direction, the reception power can be compensated by receiving the signal by two adjacent beams at the intersection and combining the outputs.
When forward link transmission is to be performed using the multi-beam scheme, using the reception powers detected by the beam power detection section 4, a beam that exhibits the maximum reception power is selected by a maximum beam weight selection section 6. A transmission beam forming section 7 transmits user data using a forward link weight coefficient corresponding to the selected beam. The signal from the transmission beam forming section 7 is converted into analog data by D/A converters 8-1 to 8-N and transmitted through transmission array antennas 9-1 to 9-N.
In executing forward link transmission using the multi-beam scheme, when the user is near the intersection of two adjacent beams, the transmission direction shifts from an optimum direction even when one of the two beams is selected and transmitted.
To solve this problem, the number of fixed beams is increased, and the resolving power in the transmission direction is raised. However, this increases the calculation amounts of the reverse link beam formers 3-1 to 3-M and maximum beam weight selection section 6.
It is an object of the present invention to provide a transmission antenna directivity control apparatus and method capable of improving the transmission direction accuracy of a transmission beam with a simple arrangement.
In order to achieve the above object, according to the present invention, there is provided a transmission antenna directivity control apparatus for controlling directivity formed by a plurality of transmission array antenna elements for a radio wave signal arrival direction to a plurality of reception array antenna elements, comprising reception multi-beam generation means for weighting and combining reception signals from the reception array antenna elements using a preset weight coefficient to generate reception multi-beams, transmission weight coefficient generation means for generating, in accordance with detection powers of the reception multi-beams from the reception multi-beam generation means, a transmission weight coefficient for transmission data in correspondence with each of transmission multi-beams corresponding to the transmission array antenna elements, and transmission multi-beam generation means for multiplying the transmission data by the transmission weight coefficient from the transmission weight coefficient generation means to generate the transmission multi-beams and supplying the transmission multi-beams to the corresponding transmission array antenna elements, the transmission weight coefficient generation means comprising power ratio calculation means for calculating a power ratio of a maximum power of the detection powers of the reception multi-beams to an adjacent power of the maximum power beam, which exhibits a larger power beam, and transmission weight coefficient calculation means for calculating the transmission weight coefficient on the basis of the power ratio from the power ratio calculation means.